Many advances in modern technology involve or are accompanied by a need to position objects extremely accurately and precisely. One arena in which this need is felt is precision machining. Another arena is precision imaging. Yet another arena is inspection and testing. One arena in which this need is particularly acute is microlithography, which generally is a technique for imprinting patterns of circuit elements on substrates. Advancements in modern microlithography systems have achieved levels of printing accuracy, precision, and resolution that were unheard of only a few years ago. For example, it is now possible to print features less than 50 nm across. To achieve such results, modern microlithography systems are extremely complex and have a large number of subsystems that must operate at or near theoretical limits of performance. These subsystems include not only illumination and imaging optics but also devices for moving objects such as reticles and substrates, and devices for measuring positions of objects such as reticles and substrates.
The particular subsystems to which this disclosure is directed is any of various “stage” devices that hold an object, such as a reticle or substrate, and that move and position the object with extremely high accuracy and precision. Referring to FIG. 11, a conventional microlithography system 100 has at least one stage, and many systems have two or more stages, such as a reticle stage 102 and a substrate stage 104 used on many types of projection-microlithography systems. The reticle stage 102 is situated and configured to hold a pattern-defining master termed a “reticle” or “mask” (generally termed a “reticle” 106 herein). The substrate stage 104 is situated and configured to hold a “substrate” 108 (e.g., a semiconductor wafer or a display panel) onto which the pattern from the reticle 106 is projected for imprinting purposes. The conventional projection-microlithography system 100 also has an illumination-optical system 110, located upstream of the reticle stage 102, that conditions a beam of lithographic light (produced by a source 112) for illuminating the reticle 106, and a projection-optical system 114, located between the reticle stage and the substrate stage 104, that receives patterned lithographic light from the reticle and projects an image of the reticle pattern onto the substrate 108. The reticle stage 102 and substrate stage 104 are independently movable in at least one direction, especially relative to the projection-optical system 114, to ensure placement of the pattern image(s) at proper location(s) and with proper alignment on the substrate 108.
Further with respect to the conventional system 100, the reticle stage 102 includes a platform (also called a “fine stage”) 102F, on which the reticle 106 is mounted, and a coarse stage 102C, on which the fine stage 102F is mounted. The coarse stage 102C is movable relative to a stage base 102B, and the fine stage 102F is movable relative to the coarse stage 102C. To achieve such motion, the coarse stage 102C is coupled to an actuator 102CA mounted to the base 102B, and the fine stage 102F is coupled to an actuator 102FA mounted to the coarse stage.
Similarly, the substrate stage 104 includes a platform (also called a “fine stage”) 104F, on which the substrate 108 is mounted, and a coarse stage 104C, on which the fine stage 104F is mounted. The coarse stage 104C is movable relative to a stage base 104B, and the fine stage 104F is movable relative to the coarse stage 104C. To achieve such motion, the coarse stage 104C is coupled to an actuator 104CA mounted to the base 104B, and the fine stage 104F is coupled to an actuator 104FA mounted to the coarse stage.
For performing highly accurate positional measurements of the reticle coarse stage 102C relative to the base 102B and positional measurements of the reticle fine stage 102F, respective interferometers 116C, 116F are used. Similarly, for performing highly accurate positional measurements of the substrate coarse stage 104C relative to the base 104B and positional measurements of the reticle fine stage 104F, respective interferometers 118C, 118F are used. Typically, each interferometer 116F, 116C, 118F, 118C shown in FIG. 11 includes multiple interferometers including measurement interferometers and reference interferometers. The interferometers 116F, 116C, 118F, 118C ensure, inter alia, co-alignment of the reticle 106 and substrate 108 with the projection-optical system 114.
As microlithography systems have evolved over the years, the mechanisms of the stages 102, 104 have become increasingly sophisticated. Much of the impetus behind evolution of microlithography systems has focused on increased throughput and better (finer) imaging resolution. To achieve these goals, modern microlithography systems have had to operate at shorter wavelengths of lithographic light than previously (which has required extensive modifications of the source 112, the illumination-optical system 110, and projection-optical system 114), and have had to incorporate many changes and refinements in the stages 102, 104. For example, many conventional stages now utilize linear and/or planar motors as actuators, and the number and sophistication of interferometers have generally increased.
Demands on the achievable motions of the stages 102, 104 have also increased. For example, as the numerical aperture (N/A) of projection-optical systems has increased, the depth of field (also called depth of focus) of these systems has correspondingly decreased. Consequently, the accuracy, precision, and numbers of degrees of freedom (DOF) of stage motions have had to be improved. (Regarding the stage motions relative to Cartesian (X, Y, Z) rectangular coordinates, up to six DOF are available: X, Y, Z, θX, θY, and θZ) Whereas, for example, reticle stages are currently available that provide motion in three DOFs (X, Y, θZ), the prospect of achieving all six DOF of motion in a single stage, especially such motions controlled to accuracy and precision in the nanometer range, has presented large technical challenges.
In addition to the above, obtaining better throughput and imaging resolution also requires stages that hold larger reticles and substrates, that accelerate and decelerate faster than previously, that move at higher velocities, and that produce less vibration. Achieving these goals also has presented large technical challenges. For example, increasing the ranges of motion and increasing acceleration, deceleration, and velocity usually results in increased size and mass of the stage and of the actuators. Hence, anything that can be done to reduce the mass of the stage without compromising stage performance would be beneficial.